What is a URL?

The address of the web

Every time you click a link, type an address in your browser, or submit a form: you're using a URL. It's so common that most people never think about what it is.

A URL (Uniform Resource Locator) is the address that tells your browser where to find a specific resource on the internet. Just like a street address tells a delivery driver where to go, a URL tells the browser which server to contact, what to ask for, and how to ask for it.

URL vs URI vs URN

You'll sometimes see these terms used interchangeably, but they have distinct meanings:

Think of it this way:

• A URN is like a person's name: it identifies them but doesn't tell you where they are
• A URL is like their home address: it identifies them AND tells you how to find them
• A URI is the umbrella term that covers both

In practice, almost everything on the web is a URL. When someone says "URI" in web development, they usually mean a URL.

Anatomy of a URL

A URL is made up of several components, each with a specific purpose:

https://www.example.com:443/catalog/courses?level=intern&sort=name#section-2
└─┬──┘  └──────┬──────┘└┬┘└──────┬────────┘└──────────┬──────────┘└───┬────┘
scheme       host     port     path            query string       fragment

Let's break this down:

Not all components are required. A web address can be just a scheme and a host:

https://example.com

Absolute vs relative URLs

URLs come in two forms:

Absolute URLs

Include the full address: scheme, host, and path:

https://www.example.com/catalog/courses

Used when linking to external sites or when the full address is needed (emails, APIs, redirects).

Relative URLs

Omit the scheme and host, using only the path (relative to the current page):

<!-- On https://www.example.com/catalog/roadmap -->
<a href="/catalog/courses">Courses</a>        <!-- Absolute path -->
<a href="courses">Courses</a>                  <!-- → /catalog/courses -->
<a href="../about">About</a>                   <!-- → /about -->

In Rails, you'll mostly use root-relative paths generated by route helpers like catalog_course_path.

Why URLs matter for developers

Understanding URL anatomy isn't just academic: it's essential for web development:

• Routing: Rails maps URLs to controller actions. Every route you define is a URL pattern.
• SEO: Search engines use URLs to understand your site's structure. Readable URLs help people and crawlers understand a page, though they are a minor ranking signal.
• Security: Sensitive data in URLs (like tokens in query strings) can leak through browser history, server logs, and referrer headers.
• Debugging: When something goes wrong, the URL is the first thing you check: wrong path? Missing parameter? Wrong protocol?
• APIs: RESTful APIs are built entirely around well-structured URLs.

# In Rails, routes define URL patterns:
# GET /catalog/courses/:slug → Catalog::CoursesController#show

# The URL tells Rails:
# - HTTP verb: GET (safe, read-only)
# - Controller: Catalog::CoursesController
# - Action: show
# - Parameter: slug = "url-anatomy"

A brief history

Tim Berners-Lee created URLs around 1990 as part of the World Wide Web. RFC 1738 first specified them in 1994; today's rules come from RFC 3986 and the WHATWG URL Standard that browsers follow.

The original design was intentionally human-readable: you should be able to look at a URL and have a rough idea of what it points to. This principle still guides good URL design today:

# Good: descriptive, readable
https://example.com/catalog/courses/url-anatomy

# Bad: opaque, meaningless
https://example.com/p?id=48372&v=2&ref=x

Summary